A problem associated with welding materials comprising austenitic stainless steel alloy base metals is that severe microfissuring or hot cracking may occur in the weld fusion zone, heat affected zone and in the base metal itself. This is particularly true where the article to be welded comprises a casting as distinguished from a wrought structure which has different grain morphology and chemical inhomogeneity.
It is believed that the microfissuring of such welds results from microsegregation when certain alloy elements, usually trace impurities, segregate along grain boundaries of either the base metal or weld material during the solidification phase of the welding process. These trace elements and segregated alloy elements in turn lower the melting temperature and therefore create a weaker film at the grain boundaries. At the same time shrinkage stresses build up and may exceed the rupture strength in the boundary film causing microfissuring.
The formation of grain boundaries and subsequent weaker boundary films is aggravated where the base metal comprises a casting. This is because certain alloy ingredients, for example silicon, are usually added to the casting alloy to lower surface tension of the molten alloy to improve its flow characteristics and therefore its castability. These ingredients also tend to lower the melting temperature along the film at grain boundaries thus adding to the microfissuring problem.
Attempts have been made in the past to control microfissuring of austenitic stainless steel welds by control of the chemistry of the filler or of the base metal. For example a filler metal that results in the formation and retention of ferrite in the austenitic weld may be used, or a delta ferrite may be added to the base metal where the ferrite acts as a crack stopper. Use of such filler metals or changes of chemistry in the base metals has not however reduced microfissuring in austenitic stainless steel alloy welds to an acceptable degree even when the weld has been carefully protected during the welding process by a shielding gas such as helium or argon or a mixture thereof.
It is therefore an object of our invention to provide for a process for substantially reducing microfissuring in welds involving fully austenitic stainless steel alloys using conventional filler metal compositions.